Brushless permanent magnet and reluctance motors can be used in many applications, since they do not require the use of commutators or brushes in supplying electrical power to the rotor of the motor. Since these components are subject to significant wear, it is highly desirable to avoid their use. The rotors of flux switching machines have salient teeth with no windings or permanent magnets. These rotors are therefore simple to manufacture and very robust. They are suitable for many applications including very high speed electrical machines.
FIG. 1 shows a flux switching machine with 8 stator teeth and 4 rotor teeth as described in U.S. Pat. No. 6,788,020. This motor contains a field winding in slots 1,3,5,7 of the stator 10 and an armature winding in slots 2,4,6,8 of the stator 10. The rotor, 11, is a salient pole rotor made from laminated steel with 4 rotor teeth, 9. This motor operates with direct current in the field winding and alternating current in the armature winding. The direct current in the field winding creates a four pole stator flux pattern which links the armature winding in a positive or negative direction as the rotor turns from alignment with stator teeth 21,23,25,27 to alignment with stator teeth 22,24,26,28. This alternating flux linking the armature generates an internal EMF in the armature. The machine can be used as a motor or generator by controlling the armature current to be in phase (motor) or out of phase (generator) with the armature EMF. This machine provides a simple and easy to manufacture structure and gives excellent control flexibility with easy variation of both field current and armature current.
The operation of the flux switching machine has been described in published papers. In a paper “Low cost high power density, flux switching machines and drives for power tools”, in IEEE IAS Annual Meeting 2003 by H. Pollock, C. Pollock, R. Walter and B. Gorti, the operation of the machine with field winding in both series and shunt configurations relative to the armature switching circuit is described. In a paper “Flux switching machines for automotive applications” by C. Pollock, H. Pollock, R. Barron, J. Coles, D. Moule, A. Court, R. Sutton, published in IEEE Transactions in Industry Applications Vol. 42 No. 5, September 2006, pp 1177-1184, the operation of the machine as a motor with bifilar armature windings is described. The flux switching motor of FIG. 1 and those described in these published papers are single phase flux switching motors, having a field winding and a single electromagnetic armature carrying alternating current.
FIG. 2 shows a further single phase flux switching machine, also from the prior art, as described in a paper “A permanent magnet flux switching motor for low energy axial fans”, Y. Cheng; C. Pollock and H. Pollock; Fourtieth IAS Annual Meeting Conference Record, Volume 3, 2-6 Oct. 2005 Page(s):2168-2175. This motor is the four pole version of a two pole machine first described in a paper “Design principles of flux switch alternator,” S. E. Rauch and L. J. Johnson, AIEE Trans., vol. PAS-74, pp. 1261-1268, 1955. The stator 30 of FIG. 2 employs four permanent magnet sections 31,33,35 and 37 interspersed between four laminated stator sections 32, 34, 36 and 38 each carrying a slot for the armature winding. As in the motor of FIG. 1 rotation of the rotor 11 causes a cyclical variation in the flux linking the armature winding and hence induces an EMF in the armature winding. The EMF is proportional to speed and unlike the machine in FIG. 1 the field flux produced by the permanent magnets cannot be altered significantly as there is no field winding. The machine of FIG. 2 therefore provides a machine of high efficiency since the magnetic field is produced without copper losses in a field winding. Flux switching machines which incorporate field windings and permanent magnets are also possible as disclosed in UK Patent Applications 0721074.3 and 0721077.6.
Operation of all the prior art single phase flux switching machines including those shown in FIGS. 1 and 2 as a motor or as a generator requires the current in the armature winding to alternate in synchronism with the internal EMF induced within the armature winding due to the field flux. Armature current would be controlled during a positive and negative conduction block, the frequency of current reversals from a positive armature conduction block to a negative armature conduction block determined by the required speed of rotation of the rotor and the magnitude of the current in each armature conduction block determined by the torque requirement of the load, or may be simply limited by the speed of rotation of the machine and the size of the internal armature EMF.
FIG. 3 shows flux plots of a single phase flux switching motor according to the prior art. In each flux plot only the field winding is excited to observe the variation in field flux with position. In FIG. 3(a) the rotor teeth are in a position where they bridge the four stator slots containing the energised field winding. In FIG. 3(b) the rotor teeth are aligned with one group of four stator teeth. In FIG. 3(c) the rotor teeth are in a position where they bridge the four stator slots containing the un-energised armature winding. In all three plots the flux linking the field winding is relatively constant irrespective of rotor position.
FIG. 4 shows a plot of the variation in self and mutual inductance in the field and armature windings of a typical prior art single phase flux switching machine such as the one shown in FIG. 1. The graph shows Line 82 which is the self inductance in pH per turn and per meter of stack length for the field winding is also the self inductance of the armature winding. Since both the field winding and the armature winding span two stator teeth and both span one rotor pole pitch the flux linking the winding due to its own current is relatively constant. The small variation with rotor angle can be explained by variations due to fringing at the edges of the teeth. The graph also shows Line 81 which is the mutual inductance between field winding and armature winding in pH, per turn per m. The mutual inductance is strongly position dependent varying from a positive maximum near to 0° to a negative maximum near to 45°.
The torque in a flux switching motor with a single armature phase is given by:
                    T        =                                            1              2                        ⁢                          i              a              2                        ⁢                                          ⅆ                                  L                  a                                                            ⅆ                θ                                              +                                    1              2                        ⁢                          i              f              2                        ⁢                                          ⅆ                                  L                  f                                                            ⅆ                θ                                              +                                    i              a                        ⁢                          i              f                        ⁢                                          ⅆ                M                                            ⅆ                θ                                                                        (        1        )            
As the windings of a flux switching machine are pitched over two stator teeth and span one rotor tooth pitch, the self inductance of the windings in the flux switching machine are relatively constant. There is therefore little torque produced by the variation in self inductance. The rate of change in mutual inductance creates the possibility for torque production so the torque in a flux switching machine with armature and field windings can be approximated by:
                    T        =                              i            a                    ⁢                      i            f                    ⁢                                    ⅆ              M                                      ⅆ              θ                                                          (        2        )            
Therefore when the rate of change of mutual inductance with respect to increasing rotor angle is positive, and if field current and armature current are both positive, then positive torque will be produced. Positive torque will act to turn the rotor to positive increasing angle.
When the rate of change of mutual inductance with respect to increasing rotor angle is negative, field current is positive but armature current is negative, then positive torque will again be produced. Positive torque will act to turn the rotor to positive increasing angle.
However, if the polarity of the armature current were opposite in each of the above situations the direction of the torque would be reversed and the negative torque would act to turn the rotor in the opposite direction. A single phase flux switching motor can therefore produce torque in either direction and rotate in either direction dependent on the timing and direction of the armature current relative to rotor position.
FIG. 5 shows the torque vs. angle for a prior art flux switching motor with a single armature phase with positive and negative armature currents showing how a continuous stream of positive torque pulses (each lasting 45°) can be produced by alternating the armature current direction. Swapping the armature current polarity in each 45° block could produce a stream of negative torque pulses driving the motor in the opposite direction.
WO 2004/025822 discloses a single phase flux switching motor in which switching of the polarity of voltage pulses applied to the armature coils of the motor can be controlled without the use of a mechanical rotor position sensor. Since a flux switching motor has field coils and armature coils, each with a pitch double that of stator teeth, the magnetic fields generated in one coil link through an adjacent coil. As a result of this overlap, there is significant mutual inductance between the armature and field coils, the mutual inductance being dependent upon the rotational position of the rotor. This enables the rotational position of the rotor to be determined by monitoring voltages induced in the field windings as a result of current flow in the armature windings. The methods disclosed in the prior art for detecting the position of the rotor without a physical position sensor on the rotor are very successful but there is no sensorless method in the prior art to confirm the direction of rotation, or more importantly to guarantee the direction of rotation of the motor from the start.
Rotor designs based on asymmetric rotors have partially addressed this problem by ensuring that if a rotor is in a position of maximum field to armature coupling in one polarity (rotor teeth aligned with one set of stator teeth), then from this position, reversal of the armature current will create a torque in the required direction to start the motor with the required direction determined by the leading edge of the rotor asymmetry. The rotor shown in FIG. 2 has an asymmetric profile which will ensure that it will rotate anti-clockwise if armature excitation polarity is reversed after holding the rotor in a parking position associated with a first polarity of armature current.
In the paper “Starting Torque of Single-Phase Flux-Switching Permanent Magnet Motors”, IEEE Transactions on Magnetics, Vol. 42, No. 10, October 2006, the authors demonstrate the problems of starting the single phase flux switching motor. The solution offered in the paper requires an asymmetric rotor and can only deliver guaranteed starting in one direction and under conditions where the position of the rotor is known at startup to ensure the correct polarity of armature current is selected. The further difficulties of guaranteeing motor starting direction without a sensor were not addressed.
Rotor asymmetry is not always desirable. It is known that adding the asymmetry to the rotor can reduce the peak torque. Furthermore, a delay time is required on starting while the rotor settles in a parked position before the other armature current direction will definitely start in the correct direction as any residual rotor oscillation could be enough to favour the wrong running direction. This is particularly a problem since the torque produced by the second armature polarity is small when the rotor teeth are aligned with the stator teeth associated with the first armature polarity. FIG. 5 shows that with excitation of −50 A the rotor could come to rest at 45°, providing there are no external torques. At this point, applying +50 A of armature current would produce a small positive torque taking the rotor in the preferred direction. However, external load could change the parking position and result in incorrect direction at start up. Once moving, it is impractical to detect and reverse the direction of rotation if a sensor is not on the shaft. As a result of these problems flux switching machines with a single electromagnetic armature have had to use a sensor to confirm direction or, if sensorless in operation, have been restricted to applications where the direction of rotation was not critical.
Flux switching machines with a field winding and three armature phases have been proposed as a way of overcoming the starting difficulties of the single phase flux switching motor. Indeed the torque output from a three phase flux switching machine excited by sinusoidal excitation can be smooth with no large torque dips or reversals as described in PCT/GB2009/001921. Three phase flux switching machines with permanent magnets creating the field are described in “Switching flux PM polyphased synchronous machines,” in Proc. 7th Eur. Conf. Power Electron. Appl., 1997, vol. 3, pp. 903-908.